The present invention is used particularly advantageously, albeit without any kind of limitation, in ultra-narrowband wireless communication systems. The term “ultra-narrowband” (UNB) is understood to mean that the instantaneous frequency spectrum of the radio signals transmitted by the terminals has a frequency width of less than one kilohertz.
Such UNB wireless communication systems are particularly suitable for M2M (machine-to-machine) applications or for applications related to the Internet of things (IoT).
In such a UNB wireless communication system, data exchanges are essentially unidirectional, in this case over an uplink between terminals and an access network of said system.
The terminals transmit uplink messages that are collected by base stations of the access network, without having to associate themselves beforehand with one or more base stations of the access network. Stated otherwise, the uplink messages transmitted by a terminal are not intended for one specific base station of the access network, and the terminal transmits its uplink messages under the assumption that they will be able to be received by at least one base station. Such arrangements are advantageous in that the terminal does not need to take regular measurements, which are intensive particularly from the point of view of power consumption, in order to determine the most appropriate base station to receive its uplink messages. The complexity is on the access network, which needs to be capable of receiving uplink messages that can be transmitted at arbitrary times and at arbitrary center frequencies. Each base station of the access network receives uplink messages from the various terminals that are within its range.
Such a mode of operation, in which data exchanges are essentially unidirectional, is entirely satisfactory for many applications, such as, for example, remote reading of gas, water and electricity meters, remote surveillance of buildings or houses, etc.
In some applications, however, it may be advantageous to be able also to exchange data in the other direction, namely over a downlink from the access network to the terminals, for example in order to reconfigure a terminal and/or control an actuator connected to said terminal. However, it is desirable to provide such a capability while limiting the impact on the cost of deployment of the access network.
To this end, the use of half-duplex base stations, i.e. base stations that may receive uplink messages and transmit downlink messages, but not simultaneously, is envisaged.
However, in such a case, it is understood that a base station that switches to a transmission mode, in order to transmit a downlink message to a terminal, is no longer available for receiving uplink messages transmitted by other terminals, such that numerous uplink messages may thus be missed.